Generally, it is very important for a sheet glass (or, a glass pane) applied to a liquid crystal display to maintain its flatness to a certain level in order to accurately realize an image. The sheet glass is prepared by a fusion method or a float method. Most existing sheet glasses (about 95% or more) are prepared by the float method. A glass produced by the float method (or, a float glass) is processed into a ribbon shape in a float bath and then cut into a predetermined size during a cutting process. In addition, a polishing process for removing fine unevenness or impurities present at the surface of the float glasses is performed.
Meanwhile, the polishing process of a glass substrate may be classified into a so-called ‘Oscar’ method where individual glass substrates are polished one by one and a so-called ‘inline’ method where a series of glass substrates are polished successively. In addition, the conventional polishing process may also be classified into a ‘single surface polishing’ where only one surface of a glass substrate is polished and a ‘both surface polishing’ where both surfaces of a glass substrate are polished.
The conventional sheet glass polishing device polishes a sheet glass by using a polishing liquid supplied onto the polishing plate while rotating a lower unit, in a state where the sheet glass is located on the lower unit (or, the lower plate) and a polishing pad of the polishing plate (or, an upper plate) is in contact with the sheet glass. The polishing pad for polishing the sheet glass in contact with a surface of the sheet glass to be polished is attached to the polishing plate of the sheet glass polishing device.
FIG. 1 is a plane view schematically showing a conventional polishing pad.
Referring to FIG. 1, a conventional polishing pad 1 has an overall disk shape and includes a central supply hole 2 prepared at the center thereof and six radial supply holes 3 arranged radially at a predetermined radius. The supply holes 2 and 3 are used for receiving a polishing liquid from the outside toward a polishing surface of the polishing pad 1. Meanwhile, a channel for regularly dispersing a polishing liquid, supplied from the polishing liquid supply holes 2 and 3, to the entire polishing surface is provided at the polishing surface of the polishing pad 1. This channel has a channel pattern with a straight form (a rectangular lattice).
However, since the polishing pad 1 rotates (in the clockwise direction or in the counterclockwise direction) in contact with a sheet glass (not shown), the polishing liquid flowing through the channel formed at the polishing surface of the polishing pad 1 is influenced by a centrifugal force. Therefore, in the conventional polishing pad 1, the rotating direction of the polishing pad 1 is not in agreement with the direction of the straight lattice-type channel pattern of the polishing surface. This causes a flux difference or irregular flow of the polishing liquid which flows through the channel formed at the polishing pad 1. Meanwhile, in case of the polishing pad 1 having such a channel pattern, if a polishing rate is high or an amount of supplied polishing liquid is great, a hydroplaning phenomenon may occur during the polishing process.
FIG. 2 is a graph showing a measured speed distribution of a polishing liquid which flows through the channel of the conventional polishing pad of FIG. 1. Here, the X axis of the graph represents an arbitrary location of the polishing pad 1, which means a direction expressed by the Roman alphabet, and the Y axis represents a flow rate (kg/m2s) of the polishing liquid.
Referring to FIG. 2, a speed deviation of the polishing liquid generated from the entire polishing surface of the polishing pad 1 is 0.6 m/s, which is very great. In other words, the flux difference of the polishing liquid is remarkable near the edge of the polishing pad 1.